In my opinion, writing is good when it respects the intelligence of the audience and also leaves nothing unambiguous. Kannukkul Nilavu executes that wonderfully well beyond several times of its time.
The plot moves steadily, not leaving too much scope for the audience to theorize, setting up reveals at appropriate times which earns the attention that it manages to keep.
Continuity is pretty good in the film, with no notable errors.
Supporting characters do a good job, genuinely contributing to the plot and not trying to squeeze in anything that doesn't fit for the film. Vijay is more a character in the film than a great hero who wants to earn some cheap whistles. He delivers a role that is completely in line with the seriousness of the film and only shines through his performance, not to mention a little singing.
The film respected Shalini, Raghuvaran and Srividya's roles, with each sticking genuinely to their role. Writing doesn't make anyone seen it of character, and doesn't make the audience hate anyone. I can be sure that I'll be revisiting this film for reasons different from why I re-watch other Vijay films: this time it won't be got fun but to admire a good setup and a good reveal in a film that rises above formulaic mass movies.
I wish we had more films like this from the early 2000s.